Magnetic Random Access Memory (MRAM) is a non-volatile computer memory technology based on magnetoresistance. One type of MRAM cell is a spin torque transfer MRAM (STT-MRAM) cell, which includes a magnetic cell core supported by a substrate. The magnetic cell core includes at least two magnetic regions, for example, a “fixed region” and a “free region,” with a non-magnetic region between. The free region and the fixed region may exhibit magnetic orientations that are either horizontally oriented (“in-plane”) or perpendicularly oriented (“out-of-plane”) relative to the width of the regions. The fixed region includes a magnetic material that has a substantially fixed (e.g., a non-switchable) magnetic orientation. The free region, on the other hand, includes a magnetic material that has a magnetic orientation that may be switched, during operation of the cell, between a “parallel” configuration and an “anti-parallel” configuration. In the parallel configuration, the magnetic orientations of the fixed region and the free region are directed in the same direction (e.g., north and north, east and east, south and south, or west and west, respectively). In the “anti-parallel” configuration, the magnetic orientations of the fixed region and the free region are directed in opposite directions (e.g., north and south, east and west, south and north, or west and east, respectively). In the parallel configuration, the STT-MRAM cell exhibits a lower electrical resistance across the magnetoresistive elements (e.g., the fixed region and free region). This state of low electrical resistance may be defined as a “0” logic state of the MRAM cell. In the anti-parallel configuration, the STT-MRAM cell exhibits a higher electrical resistance across the magnetoresistive elements. This state of high electrical resistance may be defined as a “1” logic state of the STT-MRAM cell.
Switching of the magnetic orientation of the free region may be accomplished by passing a programming current through the magnetic cell core and the fixed and free regions therein. The fixed region polarizes the electron spin of the programming current, and torque is created as the spin-polarized current passes through the core. The spin-polarized electron current exerts the torque on the free region. When the torque of the spin-polarized electron current passing through the core is greater than a critical switching current density (Jc) of the free region, the direction of the magnetic orientation of the free region is switched. Thus, the programming current can be used to alter the electrical resistance across the magnetic regions. The resulting high or low electrical resistance states across the magnetoresistive elements enable the write and read operations of the MRAM cell. After switching the magnetic orientation of the free region to achieve the one of the parallel configuration and the anti-parallel configuration associated with a desired logic state, the magnetic orientation of the free region is usually desired to be maintained, during a “storage” stage, until the MRAM cell is to be rewritten to a different configuration (i.e., to a different logic state).
A magnetic region's magnetic anisotropy (“MA”) is an indication of the directional dependence of the material's magnetic properties. Therefore, the MA is also an indication of the strength of the material's magnetic orientation and of its resistance to alteration of its orientation. Interaction between certain nonmagnetic material (e.g., oxide material) and magnetic material may induce MA (e.g., increase MA strength) along a surface of the magnetic material, adding to the overall MA strength of the magnetic material and the MRAM cell. A magnetic material exhibiting a magnetic orientation with a high MA strength may be less prone to alteration of its magnetic orientation than a magnetic material exhibiting a magnetic orientation with a low MA strength. Therefore, a free region with a high MA strength may be more stable during storage than a free region with a low MA strength.
Other beneficial properties of free regions are often associated with the microstructure of the free regions. These properties include, for example, the cell's tunnel magnetoresistance (“TMR”). TMR is a ratio of the difference between the cell's electrical resistance in the anti-parallel configuration (Rap) and its resistance in the parallel configuration (Rp) to Rp (i.e., TMR=(Rap−Rp)/Rp). Generally, a free region with a consistent crystal structure (e.g., a bcc (001) crystal structure) having few structural defects in the microstructure of its magnetic material has a higher TMR than a thin free region with structural defects. A cell with high TMR may have a high read-out signal, which may speed the reading of the MRAM cell during operation. High TMR may also enable use of low programming current.
Efforts have been made to form free regions having high MA strength and having microstructures that are conducive for high TMR. However, because compositions and fabrication conditions that promote a desirable characteristic—such as a characteristic that enables high MA, high TMR, or both—often inhibit other characteristics or performance of the MRAM cell, forming MRAM cells that have both high MA strength and high TMR has presented challenges.
For example, efforts to form magnetic material at a desired crystal structure include propagating the desired crystal structure to the magnetic material (referred to herein as the “targeted magnetic material”) from a neighboring material (referred to herein as the “seed material”). However, propagating the crystal structure may be inhibited, or may lead to microstructural defects in the targeted magnetic material, if the seed material has defects in its crystal structure, if the targeted magnetic material has a competing crystal structure to that of the crystal material, or if competing crystal structures are also propagating to the targeted magnetic material from materials other than the seed material.
Efforts to ensure that the seed material has a consistent, defect-free crystal structure that can be successfully propagated to a targeted magnetic material have included annealing the seed material. However, because both the seed material and the targeted magnetic material are often simultaneously exposed to the annealing temperatures, while the anneal improves the crystal structure of the seed material, the anneal may also begin crystallization of other materials, including the targeted magnetic material and other neighboring materials. This other crystallization can compete with and inhibit the propagation of the desired crystal structure from the seed material.
Efforts to delay crystallization of the targeted magnetic material, until after the seed material is crystallized into a desired crystal structure, have included incorporating an additive into the targeted magnetic material, when initially formed, so that the targeted magnetic material is initially amorphous. For example, where the targeted magnetic material is a cobalt-iron (CoFe) magnetic material, boron (B) may be added so that a cobalt-iron-boron (CoFeB) magnetic material may be used as a precursor material and formed in an initially-amorphous state. The additive may diffuse out of the targeted magnetic material during the anneal, enabling the targeted magnetic material to crystallize under propagation from the seed material, after the seed material has crystallized into the desired crystal structure. While these efforts may decrease the likelihood that the targeted magnetic material will be initially formed with a microstructure that will compete with the crystal structure to be propagated from the seed material, the efforts do not inhibit the propagation of competing crystal structures from neighboring materials other than the seed material. Moreover, the additive diffusing from the targeted magnetic material may diffuse to regions within the structure where the additive interferes with other characteristics of the structure, e.g., MA strength. Therefore, forming a magnetic material with a desired microstructure, e.g., to enable a high TMR, while not deteriorating other characteristics of the magnetic material or the resulting structure, such as MA strength, can present challenges.